1. Field of the Invention
The present invention relates to a new class of photoinitiators, especially suited for UV LED curable compositions.
2. Description of the Related Art
Photoinitiators are frequently used in polymerizable compositions, such as UV-curable inks, to initiate the polymerization of monomers when exposed to UV radiation. Bathochromic photoinitiators, absorbing in the region between 365 nm and 395 nm, are required to make full use of the recent development of UV-LEDs with increasing power. Thioxanthones and acyl phosphine oxides are photoinitiators absorbing in this spectral region.
Thioxanthones are prone to yellowing upon exposure, thereby forming degradation products with a limited stability. As a result, the original yellowing shifts upon storage. Especially in imaging, e.g. inkjet printing, this unstable yellowing behaviour makes control of the image tone in the final image difficult. On top of that, certain applications, predominantly packaging applications, prefer thioxanthone free radiation curable compositions.
Acyl phosphine oxides, on the other hand, result in medium volatile aldehyde type of degradation products, resulting in a background smell of the printed image, which is unacceptable in packaging applications.
Therefore, there is an increasing demand for the development of new photoinitiators, absorbing in the region between 365 nm and 395 nm, having a stable yellowing behaviour without generating medium volatile degradation products. Recent evolutions in bathochromic photoinitiators are based on carbazole derivatives.
Much effort in carbazole based initiators has been directed towards the development N-acyl oxime derivatives of bis ketocarbazoles as photoinitiators for black resists as recently reviewed by Dietliker et al. (Progress in Organic Coatings 58, 146-157 (2007) and disclosed in WO 2008/138733 (CIBA), WO 2008/138732 (CIBA), WO 2008/138724 (CIBA), WO 2007/071497 (CIBA), WO 2007/062963 (CIBA), WO 2006/018405 (CIBA), WO 2004/050653 (CIBA), WO 02/100903 (CIBA), WO 2008/075564 (MITSUBISHI CHEMICAL) and WO 2006/059458 (ASAHI DENKA).
Carbazole based Norrish type I initiators have been disclosed in JP 2007-254701 (TOYO INK), US 2003199601 (SAMSUNG ELECTRONICS), JP 63-168403 (FUJI PHOTO FILM) and EP 284561 A (CIBA)).
Bis-keto-carbazoles have been disclosed in photochemical applications as sensitizers for acyl oxime and oxime based photoinitiators in negative resist application (JP 2007-219362 (TOYO INK)) and radiation curable applications JP 2007-112930 (TOYO INK) and JP 2005-187678 (TOYO INK)). They have further been disclosed as sensitizing agents for cationic radiation curable formulations in US 2005113483 (KONICA), JP 2005-343847 (TOYO INK), JP 2000239648 (JSR) and Yamamura et al., Journal of Photopolymer Science and Technology, 13(1), 117-118 (2000)) and JP 2001-109142 (JSR)).
The carbazole initiators, disclosed in the prior art often require multistep synthesis and are often still to hypsochromic to be cured by LED curing. Therefore, there is still a need for easy accessible photoinitiators exhibiting high curing speed upon LED exposure.